The invention relates generally to light-guide films and more specifically to a mechanism for guiding light to individual cells in a group within an optically controllable metamaterial, metasurface, frequency selective surface, reflect-array or other such electromagnetic structures, all of which will be assumed in this disclosure with the term metamaterial. Edge lighting has been used for commercial signs, such as described in U.S. Pat. No. 1,139,723.
Light-guide films have been used to illuminate alphanumeric and other symbolic characters in electronic devices using an illumination source directed through the edge of the film and directed through select regions of the display surface. FIG. 1 shows a cross-sectional view 100 of a conventional light-guide planar structure 110. A transparent substrate that forms a light-guide film (LGF) 120 includes protrusions, indentations, or other structures called extractors 130 that extend from a planar surface. An illumination source 140 emits light 150 that enters an edge of the substrate 120 and through refraction and reflection exists as scattered light 160 from the extractors 130. Artisans of ordinary skill will note that light can be input into the film using any number of processes and from an edge or from a surface, and extracted or inserted using various forms of extractors.
Electromagnetic (EM) metamaterials constitute synthetic materials based on cellular elements that act to polarize electric and/or magnetic fields. Often resonant behavior is used to enhance polarization, but resonance typically leads to single frequency, or limited bandwidth use. The utility of a metamaterial could be enhanced dramatically if it were dynamically controllable over a range of frequencies. Often, the resonance is based on an electrical resonant circuit such as an inductor-capacitor circuit. Tuning can be accomplished by any capacitance, inductance, conductance or active circuit. Unless otherwise stated, the term capacitance will be employed herein but it should be known other techniques can be used.
An optimal way to control an EM metamaterial is by using photons in any portion of the frequency spectrum in which light guide film can be used, including infrared, visible and ultraviolet. This is because such photons can have a wavelength distinct from the EM radiation the metamaterial was designed for, e.g., infrared light to control a radio frequency (RF) metamaterial. Example methods for optically controlling a metamaterial has been described in U.S. Pat. No. 7,525,711 using photosensitive capacitance to modify the capacitive gap in a tunable spit-ring resonator. This technique is also described by K. A. Boulais et al.: “Tunable split-ring resonator for metamaterials using photocapacitance of semi-insulating GaAs,” Appl. Phys. Lett., 93 (4), 2008. The split-ring resonator constitutes an inductive-capacitive circuit and represents a magnetic cell in an EM metamaterial.
Other methods are described by J. Y. Chen et al.: “Comparative Analysis of Split-Ring Resonators for Tunable Negative Permeability Metamaterials Based on Anisotropic Dielectric Substrates,” Progress in Electromagnetics Research M, v. 10, pp. 25-38, 2009 and by Y. S. Kivshar: “Nonlinear and tunable metamaeterials,” in Metamaterials: Fundamentals and Applications II. A more recent example using photodiodes connected to varactors is by I. V. Shadrivov et al.: “Metamaterials Controlled with Light,” Phys. Rev. Lett., 109, 083902, 2012. In this latter work, a photodiode generates a voltage that in turn biases the varactor, which is a voltage controlled capacitance. In any of these optical schemes, a method must be employed that efficiently carries light to the photosensitive device. Another method involves using photosensitive ink such as that described by Kevin A. Boulais, et al.: “Optically Controllable Composite Dielectric Based on Photo-Conductive Particulates,” IEEE Trans on Microwave Theory and Techniques, 62 (7), July 2014 and U.S. Pat. No. 8,784,704.